This invention relates to an apparatus for the automatic adjustment of the position of working devices of shoe manufacturing machines, particularly the devices of a stretching mechanism for shoe heels and shanks which are adjusted to conform to shoe size numbers and whether right or left shoe lasts are being treated. The apparatus is particularly adapted for use with a shoe manufacturing line with a conveyor. The lasts are provided with coding recesses in accordance with which the apparatus comprising such mechanically controlled devices is adjusted. The adjustment according to size numbers of lasts has heretofore been mostly accomplished independently, by mechanism controlled by attendants of the machine. In line production in modern highly efficient manufacturing plants this adjustment requires a halt in production and also requires a high concentration on the part of the attendant, and thereby induces substantial fatigue, thus influencing unfavorably the quality and safety of the work.